The present invention relates to the refining of molten steel, and more particularly to the refining and cooling of molten steel with oily mill scale particles recovered from steel manufacturing operations conducted downstream of the refining step.
Steel is produced by refining hot metal from a blast furnace. The blast furnace hot metal contains a high carbon content and other ingredients, such as aluminum or silicon, at undesirably high levels, which must be removed by oxidation. In a typical steel refining operation, involving a basic oxygen furnace (B.O.F.), oxygen is injected into a hot molten iron contained in a vessel. The resulting reaction, between the oxygen and the carbon or other oxidizable impurities, is exothermic and generates tremendous quantities of heat, requiring the introduction of a cooling agent into the vessel in which the refining operation is conducted.
In the past, it has been conventional to cool molten steel during the refining operation by adding scrap steel or by adding iron ore pellets, both of which have drawbacks and disadvantages. Scrap steel must often be purchased on the open market, which is expensive, and scrap steel has a relatively low cooling capacity compared to an iron-oxide containing material. When iron ore pellets are to be used in steel refining, the iron ore therein must first be subjected to processing operations which are also relatively expensive. More specifically, raw iron ore contains impurities such as alumina and silica which must be removed from the iron ore before it can be used in a steel refining operation, and the removal of these impurities requires expensive processing operations, such as fine grinding.
After molten steel has been refined, it is solidified and then subjected to a succession of rolling operations during which the steel may be successively heated and cooled. During the successive heating and cooling of the steel, or during storage between various rolling operations, the surface of the steel oxidizes to form mill scale which is essentially an oxide of iron that contains about 70-75 wt.% iron. This mill scale flakes off during the handling and rolling of the steel and is recovered at the steel mill for recycling to earlier stages of the steel making process.
Mill scale is a prime material for use in steel making or blast furnace operations because mill scale is a relatively coarse, dense, waste oxide material of relatively high iron content which is low in tramp impurities such as alumina or silica.
Typically, mill scale recovered for recycling is initially subjected to a pre-screening operation to separate out the coarser particles larger than 3/16 in. (4.76 mm.) which may be charged directly into a blast furnace. The remaining, finer particles of mill scale must be subjected to a sintering operation before they can be added to a blast furnace. However, mill scale is oily, and, before it can be subjected to sintering, the oil must be removed, which is expensive.
Mill scale may be added to molten steel while the latter undergoes refining in a steel making vessel, but it is desirable that the mill scale be added in the form of an agglomerated shape. This agglomerated shape should have sufficient strength to withstand the forces to which it is subjected during handling and transportation. These forces, however, are not so severe as those to which iron oxide-containing materials are subjected when added to a blast furnace. Therefore, the relatively expensive agglomerating procedures to which iron oxide-containing materials (such as iron ore pellets) are subjected prior to introduction into a blast furnace (e.g., special curing or heat hardening operations) are not required in the case of iron oxide-containing materials added to a steel-making vessel.
Pelletizing is one agglomerating procedure which may be used without heating to produce an agglomerated shape. However, when mill scale in its as-received condition is subjected to a pelletizing operation, the pellets do not have sufficient green strength or aged strength after a reasonable length of time to hold together during normal handling and transportation operations. Green strength is that strength which is present before curing or aging under ambient or other conditions. Pellets normally increase in strength after aging for many hours (e.g., at least 24 hours).
If the mill scale is subjected to a grinding operation before pelletizing, the pellets have increased strength, but, because mill scale contains metallic particles as well as oxide particles, a grinding operation causes an undue amount of wear on the grinding equipment so that, from an expense standpoint, grinding of mill scale in order to render it acceptable for pelletizing, is not a practicable expedient.
An advantage of using mill scale for cooling purposes during a steel refining operation, compared to the cooling effects produced by scrap steel, is that the iron oxide in the mill scale reacts with the carbon and the other oxidizable materials in the molten hot metal, and this is an endothermic or heat-absorbing reaction. Thus, the mill scale cools the molten metal not only because of the temperature differential between the cool mill scale, at the time of its addition, and the hot molten metal, but also because of the heat absorbing, endothermic reaction between the iron oxide in the mill scale and the oxidizable ingredients in the molten metal.